The major objective of this project is to uncover the molecular mechanisms responsible for a variety of genetic rearrangements. The transposition-replication reaction of bacteriophage Mu is studied under this project as a model system. Critical steps in Mu transposition are a pair of DNA cleavages and strand transfers which generate a branched DNA intermediate. Efficient formation of this intermediate requires the phage-encoded MuA and MuB proteins and the E. coli-encoded HU and IHF proteins, ATP and Mg++. The MuA protein interacts with two distinct types of DNA sequences, one type of sequence is from the ends of the Mu genome while the other lies at an internal site within the Mu operator. These interactions with the donor DNA lead to formation of a stable protein-DNA complex in which the two Mu ends are synapsed by a tetramer of MuA. Next, a pair of single strand cuts are made to expose the 3' ends of the Mu sequence. This cleaved donor DNA remains tightly associated with the MuA tetramer and this complex efficiently captures a second "target" DNA molecule provided it is bound by MuB protein. A staggered cut is introduced into the target DNA and the two 5' ends are joined to the 3' ends of the Mu end sequences in a concerted DNA cutting and joining reaction. Evidence has been obtained that this reaction takes place by one-step transesterification mechanism. The MuB protein, an ATPase, selectively stimulates utilization of intermolecular target DNA molecules which do not carry Mu end sequences. The MuB protein dissociates preferentially from DNA molecules bound by MuA protein in a process that depends on ATP hydrolysis. Kinetic aspects of this energy transduction system have been studied.